This invention relates generally to electrochemical detection. More specifically, this invention relates to a carbon-based eletrochemical detection electrode.
Acidity (pH) is an important parameter measured in areas such as industrial process control, analytical chemistry, biomedical monitoring, and medical diagnosis. Conventional potentiometric acidity sensors have principally used glass electrodes to achieve acceptable sensitivity, selectivity, and lifetime. These glass electrodes, however, are generally not impervious to chemical attack or extreme pH levels. These electrodes also have other problems, in that they are generally not useful in wide ranging pressure or temperature environments, may be slow to respond, and are typically difficult to mass-produce at a reasonable cost. They can also be difficult to miniaturize, must be read by expensive meters adapted to use teraohm impedance signal transmission in order to compensate for the high resistance of a glass membrane, are not mechanically robust, and are potentially dangerous when used for food testing or in the human body.
Acidity-sensitive field effect transistors (pH-ISFETs) and potentiometric metal oxide pH electrodes (MOES) have been recently developed for acidity measurements. These electrodes can be manufactured as sensors that are small, rugged, and fairly reliable, but are not incredibly accurate. The pH-ISFETs typically rely on metal oxides as pH-sensitive gate insulator materials, such as SiO2, ZrO2, Al2O3, or Si3O4. Polarization of the gate films, thermal sensitivity, and photo induced junction leakage currents all induce significant drift of the sensor signal. The need for frequent calibration makes these pH-ISFETs unsuitable for continuous acidity testing.
A number of metal oxides such as Sb2O3, Bi2O3, PdO, IrO2, RuO2, ZrO2, and TiO2 have been investigated for use in potentiometric pH electrodes. The resultant drift of the electrode potential using these materials necessitates frequent calibration and limits their use use to short-term applications. The potential drifts over time due to the formation of intermediate valence oxides on the metal surface and interference of dissolved oxygen in the test solution. Metal oxides like PdO may have lower drift as electrodes, but exhibit serious redox interference.
Oxides of iridium (written IrOx) have exhibited favorable properties when used as potentometric pH electrodes, having significant advantages over glass electrodes and other metal oxide pH electrodes. These advantages have traditionally included low impedance, fast response even in non-aqueous solutions, good stability over a wide pH range, stability at temperatures up to 250° C., stability at high pressure, and chemical resistance. The fabrication methodology that is used has been shown to strongly influence the performance of these IrOx based pH electrodes. These fabrication methods mainly include electrochemical growth, electrochemical deposition, reactive sputtering, and thermal preparation.
Some of these IrOx electrodes exhibit good pH sensitivity and selectivity over cations, but redox interference and drift in general still represent significant problems. Improving electrode stability typically requires optimizing the preparation conditions and using surface treatments.
The potential drift, which may lead to errors in pH measurement, still remains a serious obstacle to the development of commercial pH electrodes based on IrOx. The magnitude of the potential drift seems to depend strongly on the preparation method. A mixed potential drifts and is sensitive to redox interference. The potential drift phenomena is a result of various factors, such as the oxidation state of the IrOx, the degree of hydration of the oxide, pinholes in the oxide film, and impurities in the oxide film. All of these factors are influenced by the preparation method of the oxide film and affect the long-term stability of the electrode. If the equilibrium between the different oxidation and/or hydration states is disturbed by environmental changes, the equilibrium will move, resulting in the potential drift phenomena. The drift is usually a slow process, but in some cases it can be as fast as 100 mV per hour.
It is therefore an object of the invention to develop a method for making sensors that can overcome the above-mentioned limitations.